The present invention relates to an apparatus for extrusion of mouldings from cross-linkable polymer material. The apparatus includes a first part to which the polymer material is supplied from a material container, and a second part that constitutes a moulding tool for moulding and cross-linking of said moulding.
The invention also relates to a method for extrusion of mouldings from cross-linkable polymer material in an extruder.
Methods for producing moulded articles from cross-linkable polymer material, and particularly for extruding pipes and tubing from polyethylene, are known to the art. Cross-linking, that is, the formation of spatial networks or cross-links in polymers, such as polyethylene, is nowadays generally carried out in accordance with two main principles. According to a first main principle, the so-called silane method, the formation of the spatial network is achieved chemically by means of functional groups, that is, chemically reactive side groups on the polyethylene chain participate in a condensation process which causes cross-links. According to another main principle the cross-linking is achieved by means of radicals that can either be formed by direct action of radiation energy on the polymer chain, for instance by irradiating the polymer chain with UV radiation, IR radiation or with an electron beam, or can be formed by means of additives, cross-linking agents, such as organic peroxides or azo-compounds. In this latter case, the cross-linking reaction is normally triggered by means of heat. The present invention relates to this latter type of cross-linking by means of heat.
Methods based on the transfer of heat from the tool generally have the advantage that all the peroxide will be used since the peroxide does not have time to evaporate. However, very long tools are then needed in order to allow the temperature to be raised in a degree which is sufficient to trigger the cross-linking. A consequence of this is that there is a high pressure drop in the tool, resulting in the necessity of high extrusion pressures, which in turn entails that the extruder must meet high standards. Long channels or passageways also result in frictional problems in the tool.
SE-B-324 450 teaches a method of moulding articles from a polymer that has been mixed with a cross-inking agent, such as an organic peroxide. The mixture is subjected to instantaneous compression in a pressure chamber of a ram-extruder and the compressed rod of material is transferred to a tubular extrusion moulding die for moulding and heating the mixture so as to initiate the cross-linking process. The aforedescribed frictional problems in the tool are a particular drawback in this process. Other drawbacks are that the process is discontinuous and energy demanding, particularly at the spider-leg transition from a material rod to a tubular moulding. This results in that the speed of manufacture will be very low, about 25 kg/h, particularly in the continuous manufacture of products like tubes. Furthermore, weakening welding lines may occur in the finished product, as a result of the spider-leg transition in the process.
Attempts have been made to solve the frictional problem by coating the tool with Teflon(copyright). The tool, however, becomes worn relatively quickly and must therefore be replaced at regular intervals. As described in WO 94/21441, attempts to solve the frictional problem have also been made by co-extruding the cross-linkable material with a thermoplastic surface layer having lower frictional resistance to the surface of the tool. One drawback with this solution, however, is that subsequent to extrusion, it is necessary to remove the co-extruded surface layer if this coating cannot be accepted on the finished product.
Some polymer materials and polymer mixtures, for instance polyethylene, have properties, such as high molecular weights, that make it difficult to process these materials in conventional screw extruders. Conventional screw extruders have long screws with different working zones, which can readily result in excessively high friction-generated temperatures in the extruder. It is therefore necessary to maintain a low speed of manufacture.
Conventional screw extruders comprise almost exclusively screws which feed and also melt and homogenise the material in the extruder through the combined effect of heat introduced through the cylinder wall of the extruder and created by the shearing generated by the screw. In conventional screw extruders a separate extrusion die is usually provided which is adapted to receive and mould the compressed material rod as it leaves the screw extruder. This type of conventional screw extruder also has the aforedescribed drawbacks, i.e. low speed of manufacture and weakenings in the finished product.
It is also known to construct a conventional screw extruder in the aforedescribed manner with the extrusion die forming an extension of the screw, therewith enabling weakening join or weld lines to be avoided in the end product. SE-A-8107529 describes an example of such extruders.
The object of the present invention is to set forth an apparatus and a method for the extrusion of mouldings from a cross-linkable polymer material and which will enable mouldings, and particularly pipes and tubes, to be extruded at an elevated production rate, including pipes and tubes of small dimensions, and also of good quality, i.e. with a high surface finish, without variations in dimensions, with thermal stability and with the desired degree of cross-linking.
This is achieved in accordance with the invention, with an apparatus of the kind described in the introduction and characterized by having a first part which includes a feed zone provided with an inlet, and a generally tubular outlet, and a rotatable feed screw arranged in the feed zone and functioning to compress and feed the polymer material from said inlet to said outlet with the material in an unmelted state, said feed screw having a length not exceeding the length of the feed zone.
In accordance with the invention, the aforesaid objects are also achieved with a method of the kind described in the introduction and which is characterized in that a feed screw in the feed zone is rotated so that the polymer material is fed and compressed to a tubular shape in the feed zone and therewith generating a pressure increase that is sufficient to feed the polymer material through the moulding tool and in that the polymer material leaves the feed screw in an unmelted state and is fed through the moulding tool in direct connection with the feed zone by the pressure generated.
The feed screw, which is short in relation to conventional feed screws, feeds and compresses the polymer material in an unmelted state in the feed zone such that a maximum increase in pressure is obtained at the exit end of the feed zone where the unmelted polymer material is transferred to the moulding tool. The increase in pressure obtained is sufficient to feed the polymer material through the long moulding tool, which is connected directly to the feed zone and in which the unmelted material is melted, homogenised, moulded and cross-linked.
The length of the moulding tool according to the invention is necessary in order that the polymer material will be sufficiently heated over a period of time which is required for the material to cross-link during its rapid passage, in relation to prior art techniques, through the moulding tool. According to the invention, the polymer material is processed by a moveable processing device in the feed zone, whereafter the polymer material leaves the moveable processing device in an unmelted or solid state and is transferred to the moulding tool and is fed through the die by means of the pressure generated in the feed zone. Thus, the polymer material is influenced solely by said pressure and by the applied heat as it passes through the moulding tool.
The inventive apparatus and inventive method enable the speed of manufacture to be increased by more than 100% and produce mouldings with a quality that is at least as good or even better as the quality of the mouldings produced with prior art techniques. The short feed screw that generates a high pressure in the unmelted material in the outlet end of the feed zone and the continuous advancement of the material in the process contributes to the higher rate of feed.
In one preferred embodiment of the invention, an extension of the core of the moulding tool extends through the feed screw. This is advantageous due to the fact that the core of the mould tool can be supported without spider legs, which increases the process rate and also enhances the quality of the finished article. This construction also enables that the relatively large pressure forces, occurring especially at high speed of manufacturing, are taken up by firmly securing the core extension.
Other advantageous embodiments are set forth in the following description of exemplifying embodiments and in the independent claims.